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Worldwide there is an increasing concern about the effects of power harmonics
on electrical systems.Using their knowledge of hand held test equipmentand
digital signal processing techniques Flukedeveloped these affordable, practical
tools todiagnose power harmonics problems.
What are power harmonics?
Power harmonics are caused by
non-linear loads suchas inverters (drives) and switch mode power supplies,even
electronic ballasts for fluorescent lighting.Linear loads draw current in
sine-waves at the supplyfrequency, non-linear loads do not, theseelectronically
controlled loads tend to draw current inshort pulses causing a distorted current
waveform tobe drawn from the supply.
AC theory demonstrates that non-linear currentconsumption actually consists
of a sum of current at thesupply frequency and at multiples of the
supplyfrequency. These multiples are the harmonics. Forexample if the supply
frequency is 50Hz then thesecond harmonic is 100Hz and the third harmonic
is150Hz, and so on. Because the electrical distributionsystem has a source
impedance greater than 0W, largeharmonic currents will cause harmonics to appear
onthe supply voltage. When this rises to significant levelsit may cause
additional problems.
Why are power harmonics a problem?
Symptoms of harmonics
usually show up in the powerdistribution equipment that support the
non-linearloads. There are two basic types of non-linear loads -single-phase and
three-phase. Single-phase nonlinearloads are prevalent in offices, while
three-phaseloads are widespread in industrial plants.
Each component of the power distribution systemmanifests the effects of
harmonics a little differently.Yet all are subject to damage and
inefficientperformance.
Neutral conductors
In a 3-phase, 4-wire system, neutral
conductors can beseverely affected by non-linear loads connected tosingle-phase
branch circuits. Under normal conditionsfor a balanced linear load, the
fundamental 50Hzportion of the phase currents will cancel in the
neutralconductor.
In a 4-wire system with single-phase non-linear loads,certain odd numbered
harmonics called triplens (oddmultiples of the third harmonic: 3rd, 9th, 15th
etc.) donot cancel, but rather add together in the neutralconductor. In systems
with many single-phase nonlinearloads, the neutral current can actually
exceedthe phase current. The danger here is excessiveoverheating because there
is no circuit breaker in theneutral conductor to limit the current as there are
in thephase conductors.
Excessive current in the neutral conductor can alsocause higher than normal
voltage drops between theneutral conductor and ground at the 240V
mainssockets.
Circuit breakers
Common thermal-magnetic circuit breakers
use a bimetallictrip mechanism which responds to the heatingeffect of the
circuit current. It is designed to respond tothe true rms value of the current
waveform andtherefore will trip when it gets too hot. This type ofbreaker has a
better chance of protecting againstharmonic current overloads.
A peak sensing electronic trip circuit breakerresponds to the peak of current
waveform. As a resultit won’t always respond properly to harmoniccurrents. Since
the peak of the harmonic current isusually higher than normal, this type of
circuit breakermay trip prematurely at a low current. If the peak islower than
normal the breaker may fail to trip when itshould.
Bus bars and connecting lugs
Neutral bus bars and
connecting lugs are sized tocarry the full value of the rated phase current.
Theycan become overloaded when the neutral conductorsare overloaded with the
additional sum of the triplenharmonics.
Electrical panels
Harmonics can occasionally effect
electrical panels.Panels that are designed for use with 50Hz currentmay become
resonant to the magnetic field generatedby higher frequency harmonic currents.
If thishappens a panel can vibrate and emit a buzzing soundat the harmonic
frequency.
Transformers
Where a substation has a delta-why
transformer, thereare two potential problems caused by harmonics.Firstly, single
phase non-linear loads produce triplenharmonics which algebraically add up in
the neutral.When this neutral current reaches the transformer it isreflected
into the delta primary winding where itcirculates and causes overheating which
may lead totransformer failures.
Secondly, since transformers are rated for 50Hz useharmonic currents can
cause core losses and copperlosses. Higher frequency harmonics cause
increasedcore loss due to eddy currents and hysteresis,resulting in more heating
than would occur at the same50Hz current. These heating effects mean
thattransformers need to be derated for harmonic loads orreplaced with specially
designed transformers.
Generators
Standby generators are subject to the same
kind ofoverheating problems as transformers. Because theyprovide emergency
back-up for harmonic producingloads such as data processing equipment they
areoften even more vulnerable. In addition tooverheating, certain types of
harmonics producedistortion at the zero crossing of the current waveformwhich
causes interference and instability for thegenerators control circuits.
Motors
Where there is a significant harmonic voltage
presenton the supply to a motor, certain harmonics can causea rotating field
that opposes the rotation of the motor.Although this may have little apparent
effect on therotation of the motor, it can cause overheating andsubsequent
damage to the motor.
Power factor
Harmonics is not only a major contributor to
adversepower factor, but can also cause overheating of powerfactor correction
capacitors.
Classification of harmonics
Each harmonic has a name,
frequency and sequence.The sequence refers to phasor rotation with respect tothe
fundamental (F), i.e. in an induction motor, apositive sequence harmonic would
generate amagnetic field that rotated in the same direction as thefundamental. A
negative sequence harmonic wouldrotate in the reverse direction. The first nine
harmonicsalong with their effects are listed below.
| Name |
F |
2nd* |
3rd |
4th* |
5th |
6th* |
7th |
8th* |
9th |
| Frequency |
60 |
120 |
180 |
240 |
300 |
360 |
420 |
480 |
540 |
| Sequence |
+ |
- |
0 |
+ |
- |
0 |
+ |
- |
0 | * Even harmonics disappear when
waves are symmetrical (typical for electrical circuits)
| Sequence |
Rotation |
Effects from skin effect, eddy currents,
etc.) |
| Positive |
Forward |
Heating of conductors, circuit breakers, etc. |
| Negative |
Reverse |
Heating as above + motor problems |
| Zero** |
None |
Heating, + add in neutral of 3-phase, 4-wire
system | ** Zero sequence harmonics (odd multiples
of the 3rd) are called “Triplens” (3rd, 9th, 15th, 21st, etc.)
Fluke 40 and 41 power harmonic meters
The Fluke 40 and 41
measure the voltage and current (via the 500A current transformer supplied) of
thecircuit under test. It can then display the voltage,current or power as a
waveform, a bargraph of theharmonics or as a digital display.
Fluke have designed these harmonics analysers to besimple to use, all
functions and displays are easilyaccessed via the fewest possible key presses,
so thereare no menus to work through.
The tester uses a set of nine multi-purpose screens topresent each type of
measurement (Volts, Amps orWatts) as a waveform, a relational bar chart
ofharmonics, or a series of digital (text) readouts. Onekey selects between
Volts, Amps and Watts, whilstanother selects the display type. With multiple
valuesand computations on each screen it becomes simple toaccess all the
information about the power, voltage orcurrent at the test point.
When selected a “Record function stores Max, Min,and Average readings that
can be easily accessedwith the cursor keys when on a text screen.
Specifications
Minimum input levels ______5V rms (using
VØ reference)
or 1A rms (using AØ reference)
Voltage (true rms)
Input
range__________________ 0.0 to 600.0V rms (ac + dc)
0.0V to ±933
peak
Basic accuracy (fundamental,
5-65Hz):
rms_____________________________ ±(0.3% + 2 digits)
peak,
dc___________________________ ±(2% + 3 digits)
Input impedance
_______________________ 1MW, balanced
Crest factor _______________>3.0
below 300V, 1.56 at 600V
Current with mV/A input (true rms)
Input range
_________________ 1.00 to 1000A rms (ac + dc)
1.0 to ±2000A peak
Basic
accuracy (fundamental, 5-65Hz):
rms ________________ ±(0.3% + 3 digits) +
probe specs
Crest factor_______________ >3.0 below 600A, 2.0 @
1000A
80i-500s current probe (supplied)
1 to 20A ac
___________________________ ±(5% + 0.3A)
20 to 100A ac
________________________________ ±5%
100 to 500A ac
_______________________________ ±2%
Power (watts and volt-amps) with 1mV/A
input
Range_______________________ 0 to 600kW (kVA) average
0 to 2000 kW
(kVA) peak
Basic accuracy (fundamental, 5Hz-65Hz):
Active
power__________±(1% + 4 digits) + probe specs
Harmonics accuracy (cursor
data)
Volts:
Fundamental to 15th harmonic:
Volts
_____________________________ ± (1% + 2 digits)
Phase
_______________________________________ ± 2°
16th to 31st harmonic:
Volts
______________________________±(2% + 3 digits)
Phase
_______________________________________±10°
Amps and Watts:
Fundamental to
15th harmonic:
Amps or Watts _______ ± (2% + 3 digits) + probe specs
16th
to 31st harmonic:
Amps or Watts _______ ± (3% + 4 digits) + probe
specs
Other measurement specifications
Frequency _____________________
5.00Hz-99.9Hz ±0.3Hz
Input bandwidth (-0.5dB) _____________ dc 5Hz to 2.1
KHz
Crest factor (CF range) _____________________ 1.00 to 5.00
Power
factor (PF) __________________________ 0.00 to 1.00
Displacement power factor
_________________ 0.00 to 1.00
Phase measurement range _________________ -179
to 180°
K-factor (KF) range (Model 41) _____________ 1.00 to 30.00
General specifications
Size _________ 9.2 3 3.9 3 2.5
inches (234 3 100 3 64mm)
Weight ____________________________________2.0
lb(1kg)
Battery:
Type ____________ 4 alkaline “C” cells
ANSI/NEDA-14A
IEC-LR14 (supplied)
Life __________________24 hours minimum
(continuous)
Temperature:
Operating ___________________ 32° to 122°F (0 to
50°C)
Storage ____________________ -4° to 140°F (20 to 60°C)
Temperature
coefficient ___0.1 3 specified accuracy per °C
(0 to 18°C, 28 to
50°C)
Humidity:
Operating _____________32 to 86°F (0 to 30°C) 90%
86 to
104°F (30 to 40°C) 75%
104 to 122°F (40 to 50°C)45%
Storage
_____________________________________ 90%
Altitude:
Operating
_______________________15,000 feet (4.6km)
Storage _________________________
40,000 feet (12km)
Shock and vibration ____________ Per MIL-T-28800, class
3
Electromagnetic compatibility
RF emissions ______________ EN 50081-1
commercial limits
FCC Part 15 Class B
VFG 243-1991
RF susceptibility
_____________ EN 50082-2 Industrial limits
Case ________________________
Drip-proof and dust-proof
Display
Type _______________________ Super
twisted liquid crystal
Contrast ______________________________
User-adjustable
Backlight ________________________________________
Yes
Safety
Designed for 600V measurements on industrial
power
distribution circuits
Overload protection:
Voltage or current
probe input ________ 600V maximum
Surge protection ________________ 6kV per
IEC 1010-1
Max. voltage isolation to earth __ 600V from any
terminal
Protection levels __________ IEC 1010-1, Pollution Degree
2
Installation Category III,
Material Group II, 600V
Protection class
___________________ Protection Class II as
described in IEC 1010-1, Annex
H
Includes:
- Fluke 40 HandHeld Power Harmonics Analyzer
- Fluke TL-24 Test Leads (1 Red - 1 Black)
- Fluke 80i-500s AC Current Clamp
- Fluke TP-20 Test Probes (1 Red - 1 Black)
- Fluke AC20 Test Clips (1 Red - 1 Black)
- Hard Copy User Manual
- Carrying Case
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